An organic light emitting element is an element which emits light by being applied with an electric field. A light emitting mechanism is said to be that an organic compound film is interposed between electrodes, voltage is applied to them, an electron injected from a cathode and a hole injected from an anode are recombined in the organic compound film to form a molecule in an excited state (hereinafter, referred to also as “molecular exciton”), and then, light is emitted by allowing energy to be discharged when the molecular exciton returns to a base state.
As for the type of the molecular exciton which is formed by the organic compound, those in a singlet excitation state and a triplet excitation state are considered to be possible. Herein, a case in which any one of them contributes to light emission is included.
In such organic light emitting element as described above, ordinarily, the organic compound film is formed with a thickness as low as less than 1 μm. Further, since the organic light emitting element is an element of a self-light emitting type in which the organic compound film itself emits light, the organic light emitting element does not require a backlight which has been used in an ordinary liquid crystal display. Therefore, there is a large advantage in that the organic light emitting element can be manufactured to be extremely thin and light-weight.
Further, for example, in the organic compound film of approximately from 100 to 200 nm, a period between the time a carrier is injected and the time it is recombined is only several tens of nanoseconds when a transportation rate of a carrier is taken into consideration so that light is emitted in the order of microseconds even including a process of from such recombination of the carrier to light emission. Therefore, an extremely fast response speed is also one of characteristics.
Further, since the organic light emitting element is a light emitting element of a carrier injection type, it is capable of being driven by a direct current voltage and hardly generates noises. As for the driving voltage, firstly thickness of the organic compound film is allowed to be of an ultra thin uniform film of about 100 nm, and then, an electrode material in which a carrier injection barrier is allowed to be small against the organic compound film is selected and, further, a heterostructure (double layer structure) is introduced and, as a result, such sufficient brightness as 100 cd/m2 is attained at 5.5 V (for example, refer to Non-Patent Document 1).
(Non-Patent Document 1): C. W. Tang et al., Applied Physics, Letters, 1987, Vol. 51, No. 12, pp. 913 to 915.
From these characteristics, namely, thin light-weight, a rapid response property, direct current low voltage driving and the like, the organic light emitting element has been paid attention as a flat panel display element of a next generation. Further, since it is a self-light emitting type and has a wide viewing angle, it is comparatively favorable in visibility and is considered effective as an element for use in a display panel for portable appliances.
Incidentally, as for a structure of the organic light emitting element as shown in Document 1, as a method for allowing the carrier injection barrier to be small against the organic compound film, an Mg:Ag alloy which is not only low in work function but also stable is used as a cathode, to thereby enhance an electron injection property. For this account, it has become possible to inject a large amount of carriers into the organic compound film.
Further, as for the organic compound film, recombining efficiency of the carrier has drastically been enhanced by adopting a single heterostructure such that a hole transporting layer comprising an aromatic diamine compound and an electron transporting light emitting layer comprising a tris(8-quinolinolato)-aluminum complex (hereinafter, referred to also as “Alq3”) are laminated. The reason can be described below.
For example, in a case of the organic light emitting element having a single layer of Alq3, since the Alq3 has an electron transporting property, most of the electrons injected from the cathode reach the anode without being recombined with holes and the light emitting efficiency is extremely low. Namely, in order to allow the organic light emitting element having a single layer to effectively emit light (or drive it at low voltage), it is necessary to use a material which can transport both electrons and holes (hereinafter, referred to also as “bipolar material”) while the Alq3 does not satisfy such conditions as described above.
However, when the single heterostructure as described in Non-Patent Document 1 is applied, the electron injected from the cathode is blocked at an interface between the hole transporting layer and the electron transporting layer, and then, confined in the electron transporting light emitting layer. Therefore, recombination of the carrier is efficiently performed in the electron transporting light emitting layer to attain an efficient light emission.
Further, it can be said that the organic light emitting element in Non-Patent Document 1 is characterized by a separation of functions such that transportation of the hole is performed in the hole transporting layer, and transportation and light emission of the electron is performed in the electron transporting layer. Such function separation concept has further been developed, and then, a technique in which three types of functions of the hole transportation, electron transportation and light emission are borne by different materials, respectively, has been proposed. By this technique, a material which has an inferior carrier transportation property but has a high light emission efficiency can be used as a light emitting material and, by adopting this material, the light emission efficiency of the organic light emitting element is enhanced.
A representative technique thereof is doping of a dye (for example, refer to Non-Patent Document 2). Namely, as shown in FIG. 3(a), in a single-hetero structure comprising the hole transporting layer 101 and the electron transporting layer 102 (also functioning as a light emitting layer), a light emission color of the dye 103 within a boundary region which is a light emitting region can be obtained by doping the dye 103 in the electron transporting layer 102. A case in which the dye 103 is doped in the hole transporting layer 101 side can also be considered.
(Non-Patent Document 2): C. W. Tang et al., Journal of Applied Physics, 1989, Vol. 65, No. 9, pp 3610–3616.
As compared to this, as shown in FIG. 3(b), there is a technique of a double heterostructure (three layer structure) in which the light emitting layer is interposed between the hole transporting layer and the electron transporting layer (for example, refer to Non-Patent Document 3). In a case of this technique, since the hole is injected from the hole transporting layer 106 to the light emitting layer 105 and the electron is injected from the electron transporting layer 107 to the light emitting layer 105, respectively, the recombination of the carrier occurs in the light emitting layer 105, and, accordingly, light emission having a light emission color of the material used as the light emitting layer 105 is attained.
(Non-Patent Document 3): Chihaya Adachi and three others, Japanese Journal of Applied Physics, 1988, Vol. 27, No. 2, L269–L271.
An advantage of such function separation as described above lies in a point that, by performing the function separation, it is not necessary to simultaneously impart one type of organic material with various functions (a light emission property, a carrier transportation property, a carrier injection property from an electrode, and the like) and, accordingly, a wide range of degree of freedom can be given to a molecular design or the like. (For example, it becomes not necessary to laboriously explore a bipolar material). Namely, a high light emission efficiency can easily be attained by combining materials each having an excellent light emission characteristics, materials each having an excellent carrier transportation property and the like in various ways.
From the aforementioned advantages, a concept of lamination structure itself (blocking function or function separation of the carrier) as described in Non-Patent Documents 1 to 3 is widely utilized.
In the organic light emitting element subjected to such function separation as described above, a technique of doping a coloring material is particularly effective in extension of a lifetime (for example, refer to Patent Document 1). As for factors thereof, mentioned are a smooth energy transfer to a host material or improvement of a film quality of the host material and the like. In Patent Document 1, rubrene is doped in the hole transporting layer, to thereby extending the lifetime of the element.
(Patent Document 1): JP-A No. 10-255985.
In Patent Document 1, since a coloring material is also doped into a hole transporting layer, a light emitting element has a structure as shown in FIG. 2 in which the coloring material is doped in each of an electron transporting layer and the hole transporting layer.
As described above, also in the element as shown in FIG. 2, a light emitting region is present in a boundary region 203 between the hole transporting layer 201 and the electron transporting layer 202. Therefore, both of two coloring materials of a first doping material and a second doping material which are present in the boundary region 203 cause light to be emitted.
In such manner as described above, when light emission occurs at a wavelength different from that at which light is intended to be emitted, a light emission color having a high purity can not be obtained. It is not favorable to use the element which emits light at such different wavelength in a full-color organic light emitting device which requires light having a high color purity in each of red, green and blue colors, respectively.
A problem of the present invention is to provide a light emitting element which can obtain light emission having a high color purity, and, at the same time, consistently emits light at the time of continuous driving, has a high resistance, has a long lifetime and is high in reliability.